1. Field of the Invention
This invention relates to alloys for casting in the field of denistry, and more particularly to alloy compositions for the manufacture of various cast dental products such as crowns and bridges.
2. Description of the Prior art
Noble metals are generally known as alloys for casting dental crowns but are quite expensive. A Ni-Cr alloy is commonly used. Since the melting point of this alloy is higher than 1200.degree. C., the alloy is difficulty fusible with a low-calorie heat source such as a gas oxygen mixture flame or gas air mixture flame from utility gas service. Such other measures as arc fusion or high frequency fusion are therefore widely employed for this purpose. In the latter case a centrifugal casting machine of high frequency dielectric heating type is required which is space consuming and rather expensive with attendant risks. In addition, the mold has to withstand high temperature conditions and requires normally the use of a filler or embedding material of the phosphate type which is highly resistant to temperature. However, the embedding material has too much strength and causes great inconvenience in destruction thereof at the removal of cast products.
Moreover, the filler is difficult to mix or knead and failure to knead it thoroughly results in nonuniformity of expansion coefficient during sintering. The filler further suffers from the drawback that it tends to be weathered during the course of three months or more after the manufacture thereof.
Dental crowns should not become discolored after installation nor should they injure the mating tooth or the occlusion counterpart, and further, of course, the crowns should be easily workable. A high degree of casting accuracy requires small casting shrinkage. The filler should not be attached or backed onto the surface of the casting products. There are therefore many requirements for alloys for dental casting applications.